Ants living in the hollow spines of the acacia typically feed on Beltian bodies and act as the plants' "bodyguards." The ant-acacia relationship is one of the most famous and well-studied examples of natural coevolution.

"This is really the first spider known to specifically 'hunt' plants; it is also the first known to go after plants as a primary food source," said Christopher Meehan of Villanova University, who noticed the vegetarian spiders during a field course in Mexico.

B. kiplingi is the only spider known to consume solid, vegetative plant food at all, Meehan said.

And while the spiders do occasionally prey on small invertebrates, field observations and chemical analyses of the spiders' tissue confirm that these arachnids consume a primarily vegetarian diet.

"I've done the math several times, and even the most conservative estimates point to near-total vegetarianism," Meehan said.

Nearly all of the prey that the spiders do eat are acacia-defending ant larvae.

Until now, "herbivory" in spiders simply meant that a spider occasionally consumed a bit of either nectar or pollen, Meehan explained. Pollen-feeding has only been well documented in a single orb-weaving species"”and even then only in juveniles that swallow pollen and anything else that gets entangled in their webs in the process of recycling their silk.

Nectar feeding is likely common among spiders that actively hunt instead of building webs, but this is still a distant second to carnivory in terms of their total diet.

The spiders also appear to construct their webs in less attractive plant "real estate", and to actively defend their nests against ant invaders.

Finally, they might actually imitate the ants. Indeed, young spiders in particular resemble the ants, which could possibly explain why they have escaped detection by scientists for so long despite rigorous study of the ants and acacias.

Meehan suspects the B. kiplingi spiders may also wear the ants' chemical scent, a theory he is now studying in further detail.

The discovery was published online on October 12th in Current Biology.